|27 January 2011
GSA Release No. 11-07
Director - GSA Communications & Marketing
PRE-ISSUE PUBLICATION 26 JANUARY 2011
Boulder, CO, USA – LITHOSPHERE is now regularly posting pre-issue publication content — finalized papers that have not been assigned to an issue but are not under embargo. GSA invites you to sign up for e-alerts and/or RSS feeds to have access to new journal content the minute it is posted online. Go to http://www.gsapubs.org/cgi/alerts and enter your e-mail address to manage your subscriptions for pre-issue postings, full tables of contents alerts, and more.
The following articles were posted online on 26 Jan. 2011. Topics include fault behavior in the southern Baja peninsula; pit craters and other geomorphic patterns in Iceland that may be analogous to those on Mars; 2,000 years of migrating earthquakes in China; the 2005 M8.7 Nias Earthquake; closure of the Neotethys and retreating Late Tertiary shorelines in Israel; and geochemical variations in shales in the Bastar craton.
Keywords: Baja California, La Paz, Los Cabos, Gulf Extensional province, pit craters, Iceland, Mars, North China, earthquakes, Nias Earthquake, Sumatra Earthquake, Tertiary, Arabia, Levant, Neotethys Israel, Bastar craton, Central Indian Shield.
Highlights are provided below. View abstracts for these early LITHOSPHERE articles now at http://lithosphere.gsapubs.org/content/early/recent.
Representatives of the media may obtain complementary copies of LITHOSPHERE articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published.
Non-media requests for articles may be directed to GSA Sales and Service, .
Geometry and evolution of rift-margin, normal-fault-bounded basins from gravity and geology, La Paz-Los Cabos region, Baja California Sur, Mexico
Melanie M. Busch et al., School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, Arizona 85287, USA. Published online 26 Jan. 2011; doi:10.1130/L113.1.
The Baja California peninsula is rifting away from mainland Mexico in an oblique-divergent setting. The majority of the deformation is located within the Gulf of California; however, an array of active normal faults on the southern end of the Baja peninsula, along the southwest margin of the Gulf of California, accommodates a small but important amount of rift-related deformation. Research by Melanie M. Busch of Arizona State University and colleagues characterizes the behavior of the individual faults within this normal-fault array as well as the system as a whole to understand their role in the rifting process. They conducted gravity surveys across basins created by the normal faults to estimate their depths and geometries. They also used optically stimulated luminescence dating techniques to estimate the age of the most recent activity along two of the major fault zones. The study results suggest that buried faults are present within these relatively shallow (about 0.5-3 km) basins, indicating that during the early stages of basin formation, strain was probably distributed across a number of smaller intrabasin faults. Late in the evolution of the basin, the basin-bounding faults developed, and the intrabasin faults died. Whereas the gulf-margin system contributes modestly to the total active extension across the Gulf Extensional province, it shows sustained, low-rate, active faulting and landscape and sedimentary basin evolution (including the highest topographic relief in the southern Baja California peninsula) across a fault array situated in a zone of transition in crustal thickness and material properties.
Coseismic, dilational-fault and extension-fracture related pit chain formation in Iceland: Analog for pit chains on Mars
David A. Ferrill et al., Dept. of Earth, Material, and Planetary Sciences, Geosciences and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA. Published online 26 Jan. 2011; doi:10.1130/L123.1.
Pit-crater chains -- alignments of conical to bowl-shaped depressions formed by surface collapse into a subsurface cavity -- are common topographic features on Mars and several other planetary bodies. A wide range of mechanisms has been proposed for their origin. Two rifting-related seismic events in 1975-1976 and 1978 along the Mid-Atlantic Ridge near the northern coast of Iceland, associated with the Krafla volcanic eruptions to the south, produced an array of pit chains in unconsolidated sediments overlying Holocene basalt flows. Fault scarps and extension fractures in basaltic lava flows are traceable laterally into overlying unconsolidated fluvial deposits, revealing contrasting deformation styles in the two mechanical layers. Formation of these structures in fluvial sediment was triggered by reactivation of faults and extension fractures in the underlying basalt. Pit craters are readily explained by downward "draining" of poorly consolidated material into subterranean cavities produced by fault and extension-fracture dilation in underlying cohesive material (basalt). This field-based study by David A. Ferrill of Southwest Research Institute and colleagues demonstrates that pit craters are readily explained by movement of poorly consolidated material downward into subterranean cavities produced by dilational fault and extension-fracture dilation in underlying cohesive material (basalt). Directly analogous geomorphic patterns on Mars and other planetary bodies that are visible in high-resolution surface imagery suggest that similar mechanisms of deformation and surface collapse may be at work on Mars and, potentially, other planetary bodies. Dilational faults, extension fractures, and tectonic caves on Mars, similar to those observed in Iceland, also could provide potentially large and porous reservoirs for water or water ice storage on Mars.
2000 years of migrating earthquakes in North China: How earthquakes in midcontinents differ from those at plate boundaries
Mian Liu et al., Dept. of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA. Published online 26 Jan. 2011; doi:10.1130/L129.1.
The complex spatiotemporal pattern of large earthquakes in North China during the past 2000 years indicate mechanical coupling between wide-spread fault systems. The difference between these earthquakes and the more regular plate boundary earthquakes is explained in a simple conceptual model by Mian Liu of the University of Missouri and colleagues. In this model, slow tectonic loading in mid-continents is accommodated collectively by a complex system of interacting faults, each of which can be active for a short period after long dormancy, thus producing episodic and spatially migrating large earthquakes.
Pore-fluid migration and the timing of the 2005 M8.7 Nias earthquake
Kristin L.H. Hughes et al., Dept. of Geological Sciences, University of Alabama, 201 7th Avenue, Tuscaloosa, Alabama 35487, USA. Published online 26 Jan. 2011; doi:10.1130/L109.1.
The 2004 M9 Sumatra Earthquake was one of the largest and deadliest earthquakes in history. This earthquake ruptured a 1200-km-long boundary separating the India and Eurasian Tectonic Plates. Three months later, the 2005 M8.7 Nias Earthquake ruptured what was an adjacent and locked portion of the plate boundary. Numerical models by Hughes et al. that simulate tectonic plates suggest that stress from the Sumatra Earthquake initially weakened the adjacent plate boundary, but not enough to trigger the Nias Earthquake. However, during the three-months separating the two earthquakes, water in the pore spaces of the tectonic plates slowly migrated in response to the stress. This migration caused the water pressure to slowly increase and thus weaken the adjacent and locked portion - ultimately triggering the Nias Earthquake. These numerical models also suggest that without water in the pore spaces, the gradual movements of the tectonic plates would require seven years to accumulate sufficient stresses to trigger the Nias Earthquake. This implies that water in the pore spaces advanced the timing of the Nias Earthquake by seven years. That is, without water in the pore spaces of rocks, we be would expecting the M8.7 Nias Earthquake later this year, rather than in 2005.
Retreating Late Tertiary shorelines in Israel: Implications for the exposure of north Arabia and Levant during Neotethys closure
Z. Gvirtzman et al., Geological Survey of Israel, 30 Malkhei Yisrael Street, Jerusalem, 95501, Israel. Published online 26 Jan. 2011; doi:10.1130/L124.1.
Along with the closure of the Neotethys after the middle Eocene, the paleogeography of the northern Arabian Peninsula changed significantly. Shorelines that had previously extended from Egypt eastward toward the Persian Gulf changed their course northward toward Turkey along the present-day Mediterranean coasts. Z. Gvirtzman of the Geological Survey of Israel and colleagues examine the trends of Late Tertiary shorelines from central Israel northward and documents the gradual change in shoreline direction. Based on their presented data and a compilation of additional knowledge, Gvirtzman et al. discuss the regional, gradual land exposure and suggest that northern Israel was exposed much later than central and southern Israel and that a major phase of regional uplift occurred in the early Miocene.
Evaluation of provenance, tectonic setting, and paleoredox conditions of the Mesoproterozoic-Neoproterozoic basins of the Bastar craton, Central Indian Shield: Using petrography of sandstones and geochemistry of shales
H. Wani, Dept. of Geology, Amar Singh College, Srinagar 190008, India; and M.E.A. Mondal. Published online 26 Jan. 2011; doi:10.1130/L74.1.
This article by H. Wani of Amar Singh College and M.E.A. Mondal of Aligarh Muslim University, India, provides a new understanding of what controls the geochemical variations in shales that produce contrasts between the paleoredox conditions of the Proterozoic oceans and the Proterozoic crustal composition and tectonic evolution of Central Indian Shield. These evaluations involve a wide range of observations from sandstone petrography and geochemical data of shales that, none the less, provide a mutually consistent and coherent overall picture.